Interactions between aerosol, water vapor, and solar radiation

A quantitative understanding of how clouds, aerosols and atmospheric gases affect the solar radiation absorbed by earth's climate systems is still largely unknown. This dissertation makes calibrated and precise (<1%) measurements of broadband and spectral solar radiation, and integrates these measurements with state-of-the-art radiative transfer models to clarify the roles of water vapor and aerosols in the solar heating of the atmosphere and ocean.;The first part of the dissertation uses instantaneous surface, aircraft and satellite broadband (0.2--2.8 mum) solar flux measurements collected during the Central Equatorial Pacific Experiment to establish that the atmospheric absorption in this moist region is 20% +/- 2% (89 +/- 7 W m -2) of the 441 W m-2 incident at the top-of-the-atmosphere. A radiative transfer model containing simultaneously observed water vapor profiles accurately predicts the instantaneous surface irradiance over a range of 35--50 kg m-2 of column water vapor. Precise 0.94 mum direct solar transmission measurements collected during the ARM Enhanced Shortwave Experiment validate correlated-k atmospheric absorption calculations to within 5% at the center of this strong water vapor vibration rotation band. Together, these studies have significantly focused an active debate regarding the role of water vapor in regulating atmospheric solar absorption.;The second part of the dissertation used two laboratory-calibrated multi-spectral photodiode radiometers during the Indian Ocean Experiment (INDOEX) to accurately quantify surface aerosol forcing, the aerosol-induced reduction of surface solar radiation. The global and diffuse irradiance data agree to within 5 W m-2 of results calculated by a Monte Carlo radiative transfer model that assumes an aerosol consistent with simultaneously measured aerosol properties. The monthly mean 0.4--0.7 mum forcing for the region is -7.6 +/- 1.5 W m-2 during 1998 and -16.0 +/- 1.5 W m-2 during 1999. These precise estimates serve as a ground-truth for the broad scope of surface aerosol forcing studies conducted during INDOEX. Extrapolated to the broadband, the forcing is -13 W m-2 and -29 W m-2 for 1998 and 1999 respectively. Most of the surface cooling is caused by an average 0.5 K/day heating of the lower troposphere. Three years (1996--1998) of ship aerosol optical depth measurements find a surface forcing gradient of -16 W m-2 between the Arabian Sea and the South Indian Ocean.